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BAGHDAD, Iraq (CNN) -- A string of car bombs exploded Thursday in the Iraqi city of Balad, killing at least 62 people and wounding more than 70 others, a police official in Tikrit told CNN.

The bombings took place about 6:30 p.m. in the center of the mixed Shiite and Sunni town about 50 miles (80 kilometers) north of Baghdad. They detonated about 10 minutes apart, two of them in the same location.

Thursday's blasts were the most deadly single-day string of bombings since September 14, when more than 100 people were killed in Baghdad in strikes claimed by Al Qaeda in Iraq, the group led by Abu Musab al-Zarqawi.

Earlier, the U.S. Marine Corps reported that five U.S. Army soldiers were killed Wednesday in Ramadi by a roadside bomb.

The soldiers were "assigned to 2nd Marine Division, II Marine Expeditionary Force (Forward)" and died while "conducting combat operations."

The deaths bring the number of U.S. troops killed in the Iraq war to 1,936.

The Pentagon said Thursday that 21 U.S. troops were killed by hostile fire in Iraq over the previous week. So far in September, 52 U.S. troops have died in Iraq.

In Washington on Thursday, the top U.S. military commander in Iraq said that the next 75 days would be "critical" in determining whether there will be a reduction in U.S. forces there.

Gen. George Casey, testifying before the Senate Armed Services Committee, said "reducing the visibility and ultimately the presence of coalition forces as we transition to Iraqi security self-reliance remains a key element of our counter-insurgency strategy."

Sectarian civil strife has persisted in Iraq, with a largely Sunni Arab insurgency targeting the Shiite-Kurdish power structure and Shiite Arabs themselves.

President Bush predicted Wednesday that insurgents would launch a new wave of attacks aimed at disrupting upcoming Iraqi elections, but that "our troops are ready for it." (Bush discusses Iraq gains -- 7:59)

In Iraq on Thursday, gunmen killed at least nine Iraqis, including four police officers, and wounded 11, police said.

Attackers opened fire on a carload of teachers near Baquba, killing one of them. The three other teachers in the car survived the attack.

The incident occurred a few days after insurgents dressed as police shot and killed six teachers at an elementary school in Babil province, south of Baghdad.

Two high-ranking police officials from the town of Balad were shot dead Thursday in a drive-by shooting in Jami'a neighborhood of western Baghdad, police said.

Attackers targeted a police patrol in the Jihad neighborhood in western Baghdad, killing two police officers and wounding three others.

A separate attack on a minibus in the Mushtal neighborhood of southeastern Baghdad killed two employees of al-Shaab International Stadium and wounded seven others.

Two people were killed and third was wounded when gunmen opened fire at al-Mustafa bakery in the Dora area in the southern Baghdad.

General: Troop reduction possible

Casey told U.S. lawmakers he'll have a better idea about how the military will proceed after this autumn's political period -- the October 15 constitutional referendum and the mid-December election for a new government. (Full story)

"I do think that the possibility for condition-based reduction of coalition forces still exists in 2006," he said.

Casey testified that Iraqi security forces were increasing in size and in ability in the past year.

Casey said there is no timeline for a reduction, and that it would depend on the capability of Iraqi forces to take over security duties.

He said "we expect to have 60,000 to 70,000 more Iraqi security forces available for referendum security than we had in the January elections."

Gen. Richard Myers, the chairman of the Joint Chiefs of Staff, warned lawmakers that an immediate withdrawal of U.S. troops would embolden al Qaeda and that "surely the next 9/11 would be right around the corner."

But Casey said many Sunni Arabs will take part in the constitutional referendum but that they won't be able to muster the numbers to defeat it.

"Sunni Arabs who boycotted January's election remain committed to participating in both the referendum and the election," he said. "This is a significant step forward."

Casey also said the Iraqi-Syrian border is not under U.S. control.

The border with Iraq's northwestern neighbor is a notorious transit point for foreign fighters.

But Casey said U.S. and Iraqi forces have been working since April to quash the insurgency there.

Gen. John Abizaid, the head of U.S. Central Command, told lawmakers the war "has moved to the west," calling that "a good indicator that Iraqi and U.S. forces are having an effect elsewhere."

Copter crash blamed on pilot error

"The amount of infiltration across the Syrian border remains a concern, but it's down," he said.

Pilot error has been blamed for a January helicopter crash in Iraq that killed 30 Marines and one sailor on the deadliest day of the war, Pentagon officials said Thursday. (Full story)

Officials said the crew of the Marine Corps CH-53E Sea Stallion became confused in cloudy conditions at night and flew the helicopter into the ground.

They said the crash was not due to hostile fire or mechanical failure. Another helicopter flying nearby was not damaged and landed safely.

The helicopter was taking troops to western Iraq to guard polling places ahead of elections when it went down in the desert.

Other developments

A federal judge ruled Thursday that additional photographs showing abuse in Abu Ghraib prison must be released, despite government concerns that they could further damage America's image. (Full story)

The Iraqi Islamic Party, a Sunni Arab political group, Thursday denounced a raid on the Baghdad house of a top Sunni official, Dr. Adnan al-Dulaimi, secretary-general of the People of Iraq Conference. A U.S. military official said the action came after solid intelligence reports and resulted in detentions and the seizure of munitions.